1. Field of the Invention
The present invention relates to a method of starting up operation of a fuel cell at a low temperature at the freezing temperature of water or less. The fuel cell includes an electrolyte electrode assembly and separators sandwiching the electrolyte electrode assembly. The electrolyte electrode assembly includes a pair of electrodes and an electrolyte interposed between the electrodes.
2. Description of the Related Art
Generally, a solid polymer fuel cell employs a membrane electrode assembly (MEA) which includes an anode and a cathode), and an electrolyte membrane interposed between the anode and the cathode. The electrolyte membrane is a polymer ion exchange membrane (proton exchange membrane). The membrane electrode assembly and separators sandwiching the membrane electrode assembly make up a unit of a fuel cell for generating electricity. A predetermined number of the fuel cells are stacked together to form a fuel cell stack.
In the fuel cell, a fuel gas such as a gas chiefly containing hydrogen (hydrogen-containing gas) is supplied to the anode. The catalyst of the anode induces a chemical reaction of the fuel gas to split the hydrogen molecule into hydrogen ions (protons) and electrons. The hydrogen ions move toward the cathode through the electrolyte membrane, and the electrons flow through an external circuit to the cathode, creating a DC electric current. A gas chiefly containing oxygen or air (oxygen-containing gas) is supplied to the cathode. At the cathode, the hydrogen ions from the anode combine with the electrons and oxygen to produce water.
In the fuel cell, it is required to suitably humidify the electrolyte membrane (polymer ion exchange membrane) to keep the ion conductivity of the electrolyte membrane. At the cathode, water is produced by the chemical reaction. Thus, in starting operation of the fuel cell in the freezing environment, i.e., at the freezing temperature of water or less, it is likely that water in the fuel cell freezes, and electrochemical reaction is not efficiently carried out.
For example, according to the disclosure of Japanese laid-open patent publication No. 2000-512068, electrolyte membranes such as NAFION (registered trademark) of Dupont, and the experimental membrane (product No. XUX 13204.10) of Dow have sufficient ion conductivity to induce an electrochemical reaction in the fuel cell even at the temperature of −20° C.
Japanese laid-open patent publication No. 2000-512068 suggests that these membranes are advantageously used in a method of starting up a power generation apparatus (fuel cell) at a low temperature below the freezing temperature of water. The power generation apparatus includes a fuel cell stack connectable to an external electrical circuit for supplying electricity to the external electrical circuit. The fuel cell stack includes at least one fuel cell. The fuel cell has a membrane electrode assembly including a cathode, an anode, and a water permeable ion-exchange membrane interposed between the cathode and the anode. At least part of the membrane temperature assembly has a temperature below the freezing temperature of water. The method includes a step of supplying electricity from the fuel cell stack to the external circuit such that part of the membrane electrode assembly exceeds the freezing temperature of water.
FIG. 7 shows relationship (plot X) between the voltage of fuel cell stack and the time (minutes), and relationship (plot Y) between the core temperature of the fuel cell stack and the time (minutes) according to the method of Japanese laid-open patent publication No. 2000-512068. The fuel cell stack includes four fuel cells. Operation of the fuel cell stack is started at the core temperature of −23° C.
According to the disclosure of Japanese laid-open patent publication No. 2000-512068, the passage of hydrogen is closed by water or ice, for about four minutes after operation is started. Therefore, hydrogen does not flow sufficiently for the first four minutes. When the fuel cell is connected to a load of 50 ampere four minutes after starting the operation, the fuel cell can output the electrical current of about 45 ampere initially. However, the output current is decreased to about 15 ampere in a short period of 8 seconds due to freezing of water produced during power generation.
As described above, according to the disclosure of Japanese laid-open patent publication No. 2000-512068, it is possible to start operation of the fuel cell below the freezing temperature of water. However, it is difficult to start the desired operation of the fuel cell smoothly due to the ice that is present at the time of starting operation, and the frozen water produced during operation of the fuel cell.